An Ampersand.js developer designs and implements web applications using Ampersand.js, a highly modular JavaScript framework. The developer uses the framework's flexibility to create custom web applications, integrating various modules as needed. This includes managing data with Model and Collection modules, controlling application flow with the Router module, and creating interactive UI with the View module. The developer also tests, debugs, and optimizes the application for performance. Additionally, they may collaborate with a team, follow agile development methodologies, and use version control systems like Git. Their ultimate goal is to create efficient, maintainable, and user-friendly web applications.